1. Field of the Invention
The present invention relates to a semiconductor laser element, and in particular to a semiconductor laser element (typically a laser diode) which has an oscillation wavelength of about 800 nm. Hereinafter, the laser diode may be referred to as an LD.
2. Description of the Related Art
Currently, the laser machining techniques are widely used in the shipbuilding, heavy industries, aerospace industries, and the like as well as the automobile industries and electronic manufacturing industries since the laser machining techniques have more advantages than the other machining techniques. For example, the laser machining techniques enable complex and fine machining, and do not impair the material strength.
Recently, the use of solid-state lasers using a semiconductor laser as an excitation light source (which are hereinafter referred to as LD-excited solid-state lasers) has been spreading. For example, in some Nd:YAG lasers which emit infrared light having a wavelength of 1,064 nm, 808-nm band GaAs/AlGaAs-based LDs are used as excitation light sources.
However, since the active layers in the GaAs/AlGaAs-based semiconductor laser elements contain aluminum, which is prone to oxidation, the characteristics of the semiconductor laser elements can suddenly deteriorate. According to a technique which has been proposed for overcoming this problem, a light-emission region is formed of a material which does not contain aluminum, and cladding layers are formed of AlGaAs in order to prevent deterioration of the temperature characteristics.
Specifically, rapid deterioration of the characteristics caused by oxidation of aluminum is prevented by forming an active layer of InGaAsP and optical waveguide layers of GaInP. In addition, the so-called separate-confinement heterostructure (SCH) is used. In the SCH, a carrier-confinement region and a light-confinement region are separately arranged. Thus, it is expected that a semiconductor laser having high optical density and being superior in output characteristics and reliability is obtained, as indicated in “Highly Reliable Operation of High-Power InGaAsP/InGaP/AlGaAs 0.8 μm Separate Confinement Heterostructure Lasers”, T. Fukunaga et al., Japanese Journal of Applied Physics, vol. 34, No. 9B, pp. L1175-1177 (1995).
When the semiconductor laser element disclosed in the above document is produced, crystal defects can be produced in a GaInP layer during growth of a GaInP crystal on a GaAs layer by MOCVD (metal organic chemical vapor deposition). Similar crystal defects can also be produced in a AlGaInP layer, which is a similar type of material to GaInP. For example, it has been confirmed that elliptical hillocks are produced as crystal defects when a GaInP crystal is grown on a (100) face of a GaAs crystal.
Although the influence of the hillocks on the laser characteristics has not yet been sufficiently studied, it is considered that when an active layer of InGaAsP (which is a quatenary mixed crystal the composition of which is not easy to control) is formed on a GaInP layer having hillocks, the hillocks can affect the quality of the active layer.